It is conventional in underground coal mines and other mines to load the coal or other mineral at the mine face from a continuous miner machine which digs the mineral and dumps it onto a shuttle car which when loaded travels toward the belt conveyor, deposits its load on the belt conveyor and returns to the continuous miner for another load. This belt conveyor, called a subbelt, takes the mineral through the secondary mine tunnels and deposits it onto a transfer belt in the primary tunnels. The transfer belt transports the mineral onto the main belt in the main mine entry tunnel. The main belt removes the mineral from the mine for cleaning, sorting, and sizing operations.
Throughout the entire system of belt conveyor tunnels (workers use a different tunnel system to get to their working area) the belt conveyor passes through an area seldom higher than the thickness of the mineral seam. Since the belt conveyor tunnel is only intended to transport the mineral, and not provide height for mining employees the ceiling height is kept low to avoid costly removal of rock, earth, etc. Thus the belt conveyor passes through a confined area.
There is a constant need for mining employees to move safely and conveniently about the belt conveyor tunnels in performing their many duties such as belt checking, changing of rollers, or any necessary maintenance. They are also required to inspect and service conveyor belts and their supports, cables, roller arms, and electric service; to lend assistance to a coal mine employee on the other side of the belt, and at times to enter a perpendicular mine tunnel when the employee is on the side of the conveyor away from the entrance of the tunnel. These matters sometimes require them to cross over or under the moving belt conveyor.
In the present state of the art this is an unsafe thing to do. Under the conveyor belt are the hazards of conveyor belt support cable collapse, interfering cables, pulleys, wires, and minimal clearance between mine floor and bottom of conveyor belt. Over the belt are the hazards that moving coal may strike the coal mine employee, clothing may get caught on the belt and its moving parts, and the employee might fall onto the conveyor belt.
Conveyors for coal or like minerals must be installed so that the full load carried by the upper flight clears the mine roof. The spacing of the conveyor above the mine floor is not of much significance as long as the lower flight finds a return path. Mine floors generally follow the bottom of the seam and are not necessarily level. It is common practice, therefore, at least in coal mines, to suspend the conveyor from the mine roof. This is accomplished by boring holes in the mine roof and fixing the upper ends of roof bolts therein, the lower ends being attached to the conveyor frame. Those roof bolts are not necessarily the same roof bolts used to bolt together the various rock strata in the mine roof. A belt conveyor of the type generally employed in coal mines comprises an endless belt the upper flight of which is supported at intervals by groups of three rollers comprising a central horizontal roller and side or troughing rollers inclined upwardly and away from the center roller on each side to shape the upper flight of the belt into a shallow trough. Those roller groups are each mounted on a transverse roller tie bar which passes beneath the upper flight of the belt. The outboard ends of those tie bars are attached to the lower ends of the conveyor-supporting roof bolts. A wire rope or cable running along each side of the conveyor is also attached to the outboard ends of those tie bars, maintaining the spacing between them while allowing some lateral movement to compensate for uneven loading of the belt.